1. Field of the Invention
The preferred embodiments are directed to amplification of low magnitude electrical signals generated by low impedance sources, and more particularly, amplification of low voltage signals generated by position sensors such as those used to measure movement of an actuator of a scanning probe microscope (SPM).
2. Discussion of the Prior Art
A scanning probe microscope, such as an atomic force microscope (AFM) operates by providing relative scanning movement between a measuring probe and a sample while measuring one or more properties of the sample. A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15 is coupled to an oscillating actuator or drive 16 that is used to drive probe 14, in this case, at or near the probe's resonant frequency. Commonly, an electronic signal is applied from an AC signal source 18 under control of an AFM controller 20 to cause actuator 16 to drive the probe 14 to oscillate, preferably at a free oscillation amplitude Ao. Probe 14 is typically actuated toward and away from sample 22 using a suitable actuator or scanner 24 controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe. Moreover, though the actuator 24 is shown coupled to the probe 14, the actuator 24 may be employed to move sample 22 in three orthogonal directions as an XYZ actuator, i.e., both Z motion, and X-Y scanning motion such as a raster scanning.
For use and operation, one or more probes may be loaded into the AFM and the AFM may be equipped to select one of several loaded probes. Typically, the selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 17 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. Commonly, controller 20 generates control signals to maintain a constant force between the tip and sample, typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
Commonly, actuator 24 is a piezoelectric tube (often referred to herein as a “piezo tube”) or flexure that is used to generate relative motion between the measuring probe and the sample surface. A piezoelectric tube is a device that moves in one or more directions when voltages are applied to electrodes disposed inside and outside the tube. Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
Piezoelectric tubes and other types of actuators are imperfect. For example, a piezo tube often does not move only in the intended direction. When a Z actuator, for instance, is commanded to move in the Z-direction (by the application of an appropriate voltage between the actuator's electrodes), the Z actuator moves not only in the Z direction, but in the X and/or Y directions as well. This unwanted parasitic motion limits the accuracy of measurements obtained by scanning probe microscopes. The amount of this parasitic motion varies with the geometry of the tube and with the uniformity of the tube material but typically cannot be eliminated to achieve the accuracy required by present instruments.
More particularly, with respect to movement in the intended direction, the ideal behavior would be that the actuator move in exact proportion to the voltage applied. Instead actuators, including piezo tubes, move in a non-linear manner, meaning that their sensitivity (e.g., nanometers of motion per applied voltage) can vary as the voltage increases. In addition, hysteresis and creep often compromise the intended motion. Most generally, the response to an incremental voltage change will depend on the history of previous voltages applied to the actuator. These effects, thus, can cause a large prior motion to affect the response of a commanded move, even many minutes later.
Similarly, vertical measurements in scanning probe microscopy are typically calculated mathematically by recording the voltage applied to the Z actuator and then multiplying by the actuator's calibrated sensitivity in nm/V. However, as mentioned previously, this sensitivity is not constant and depends on the previous voltages applied to the tube. Using the voltage applied to the tube to calculate the vertical motion of the tube therefore will always result in an error with respect to the actual motion. This error can translate directly into errors when measuring and imaging surface topography of a sample and performing other metrology experiments. These issues have been addressed specifically for the case in which the probe assembly and device of the AFM is coupled to the actuator (i.e., the case in which the probe assembly moves in three orthogonal directions, for example, in the cases cross-referenced below), as well as when the actuator is coupled to the sample.
Methods of monitoring the motion of the probe or sample when driven by a SPM actuator have been implemented in an attempt to compensate for this parasitic X and Y error, with mixed results. The devices are typically calibrated by applying a voltage to the X-Y tube and the Z tube, and then measuring the actual distance that the sample or probe travels. Thus, the position of the actuator is estimated by the voltage that is applied to the X-Y tube and the Z tube. However, correcting for the (X,Y) position error introduced by the Z actuator on the probe or sample is difficult because it requires additional calibration steps and more complex circuitry to determine the correct voltage to apply to, the Z tube or to the X-Y tube.
Some SPMs attempt to improve accuracy by using an auxiliary displacement sensor to actively monitor actuator movement and adjusting the voltage to the appropriate SPM actuator(s) to cause the actuator to move to the desired manner. Several auxiliary displacement sensors, sometimes referred to as position sensors, have been proposed for monitoring actuator movement, including Linear Variable Displacement Transducers (LVDTs), capacitance sensors, strain gauge sensors, and optical displacement sensors (ODSs).
Such SPM actuator position signals generated by one of these types of displacement sensors are low voltage signals that must be amplified before they can be utilized by the operator or otherwise further processed (e.g., data acquisition and control). One challenge in this regard is that these weak voltage signals are susceptible to the adverse effects of electrical interference, such as “pick-up” and other miscellaneous noise, especially as they are transmitted over lengths of wire or cable. As a result, given their robust nature, sensors that produce differential outputs are often employed along with differential amplifiers. Differential amplifiers amplify the difference between two input signals, and operate to restore the original signals as long as the common mode signals are not too large. In one implementation, a detection system 30 includes strain gauge resistors or sensors 34 disposed on an SPM actuator (not shown) in a conventional fashion to monitor position and translation of the actuator. Typically, a differential output is provided by configuring the resistors 34 in a Wheatstone bridge 32, as shown in FIG. 1. In this case, an instrumentation amplifier 36 is often employed to amplify the low voltage differential signals generated by the bridge 32.
An instrumentation amplifier is an arrangement of operational amplifiers that provides programmable high gain, low offset, reasonably low noise, particularly at high gain settings, and high rejection of common mode signals, i.e., high rejection of identical signal components on both the “+” and “−” inputs of the instrumentation amplifier (high common mode rejection ratio (CMRR)), for example, due to a difference in ground between the signal source and the receiver.
Although widely used, including in the SPM environment, instrumentation amplifiers (IAs) are not ideal for many applications because IAs have relatively high power dissipation. More particularly, the amplifier is preferably placed in close proximity to the source of the low voltage signals, i.e., the position sensor (e.g., a Wheatstone bridge connected strain sensor) in this case, which oftentimes requires placing the amplifier in the AFM head. Though this typically operates to maintain a high CMRR and good immunity from electrical interference, the arrangement is often problematic because the aforementioned associated high power dissipation. The instrumentation amplifier is powered by the voltage source ±V, which typically is about 15 volts (thus, 30 volts), resulting in a typical power dissipation of 300 mW for instrumentation amplifiers optimized for low input voltage noise. Because the AFM head houses a majority of the AFM's thermally sensitive precision components, crippling thermal drift effects can be introduced into the system when including an instrumentation amplifier in or near the head. Because resolution of these instruments is on the nanoscale and below, thermal drift is a problem that can severely compromise the usability of the data obtained by an AFM.
Notably, another drawback with instrumentation amplifiers is that, if the required amplification is modest, the amplifier tends to have relatively high noise and degraded common mode rejection. The gain required to achieve the level of signal desired in AFM applications employing strain gauges and other types of sensors is dependent on, for example, the amount of strain experienced by the gauges and their sensitivity (gauge factor, i.e., the amount the resistance changes for a given strain). In this case, the required gain is relatively low given the performance of the strain gauges used which are preferably semiconductor-type strain gauges that have a limited range of detectable strain for which they are linear. To get the lowest possible noise and the best dynamic range (i.e., biggest signal at small actuator extensions), the system should be set up so that the “rail” of the amplifier is just reached at the maximum extension of the scanner. Typically, the gain required to amplify the maximum strain that produces linear operation of the semiconductor strain gauge is about 50, yielding an output of about +/−10 volts. As a result, with relatively modest gain requirements, noise and common mode rejection when using an instrumentation amplifier is not ideal.
Further with respect to the Wheatstone bridge, and similar in this regard, for the nominal condition, it is important that the bridge be balanced, i.e., produce zero differential voltage when the actuator is at its midpoint position. Otherwise, the amplifier operates to amplify some offset, thus minimizing its range of detectable outputs. When using an instrumentation amplifier, the offset may be so large that the applied gain saturates the amplifier, thus corrupting the resultant output signals. Since the elements of the Wheatstone bridge are in general not precisely equal in nominal resistance due to manufacturing variations, a balancing circuit is often provided. Three examples of such circuits are provided in FIGS. 3A-3C. In each circuit, 40, 42 and 44, respectively, a variable resistor, VR1, can be manipulated to balance the bridge in an effort to ultimately avoid amplifier saturation. However, each of these systems requires some sort of control knob located near the strain gauge which most often is quite inconvenient (real estate on an AFM is at a premium) or requires, for example, a type of digitally controlled potentiometer which can create problems given the extra signal needed to control it (i.e., typically requires an extra wire to the head which can cause troublesome electrical interference or coupling of mechanical vibrations). Moreover, each of these circuits 40, 42, 44 tend to cause drift, thus placing the designs at high risk to compromise instrument precision. For these reasons, an alternate scheme to compensate for bridge imbalance was desired.
As a result, and still further in this regard, for many applications in SPM, it is preferable to transmit the acquired signals (data, position, etc., for example) to a remote location to, for example, minimize heat dissipation in the head of the AFM. However, signals carried over cables to a remote amplifying stage are particularly susceptible to electromagnetic interference, as suggested previously. As a result, it was desired to have a balanced, differential, low impedance current mode amplified signal given that such signals can be readily carried over cables to a second amplifier or data acquisition system in a way that is relatively immune to electromagnetic interference. In this case, it would also be highly desirable to have a way to remotely compensate for imbalance of the low impedance source (e.g., a Wheatstone bridge accommodating an arrangement of strain gauge sensors) without requiring physical access to the first stage amplifier. As a result, additional wires and power dissipating elements would be minimized in the first stage of the amplifier.